The redundancy point is true. The tandem drum brakes are less to do about heat dissipation than stopping your hands cramp up on a long descent. Often you just actuate the rear drum to provide constant drag on a descent.
For anyone interested, specialist tandem outfitters used to shave off the heat dissipation fins on old-style Arai drum brakes. The point of said brakes was not to dissipate heat, but to stop the tandem. It is true that very old tandems with rim brakes had heat-dissipation problems.
http://www.bikeforums.net/tandem-cycling/52661-shaving-arai-...